1. Field of the Invention
The present invention relates to improved circuit interrupting devices, and more particularly to an improved high-voltage fuse, the condition of which is readily ascertainable without the use of special instruments or the removal of the fuse from its mounting.
2. Description of the Prior Art
High-voltage fuses of various types are well known. Such fuses contain both a fusible element which fuses, (or melts, vaporizes, or otherwise disintegrates) in response to an over-current condition and facilities for extinguishing the high-voltage arc which will be struck upon such fusing. The arc-extinguishing facilities generally include a movable component, often called an arcing rod or arcing terminal, which is normally restrained in a first position by attachment to an end of the fusible element as long as the fusible element does not fuse. The arcing rod is movable through or past a region or area which contains an arc-extinguishing medium or material. All of these items--the fusible element, the arcing rod, and the arc-extinguishing material--are normally contained within a hollow insulative housing. The housing is made of an insulative material having sufficient dielectric strength and length in view of the voltage at which the fuse is intended to be used and sufficient mechanical strength to withstand the handling incident to installation as well as the internal pressures resulting from the arc being struck at the time of circuit interruption. The ends of the housing are usually closed with metallic end caps or ferrules. These end caps are, when the fuse is in place, electrically connected to and mechanically held by an appropriate fuse mounting, thereby serially inserting the fuse in a circuit.
Upon an over-current condition arising in the circuit in which the fuse is inserted, the fusible element fuses and an arc is struck. This fusing permits facilities, usually a spring-like member, to move the arcing rod through or past the arc-extinguishing material. The usual arrangement of these types of fuses is such that the arc terminates on the arcing rod at or near the point of the former rod/element attachment and at or near a remote point formerly occupied by another end of the fusible element. The arc is thus drawn or elongated by the now-moving arcing rod so that it interacts with the arc-extinguishing material. Turbulent, cooling, deionizing gases are generated by this interaction between the arc and the arc-extinguishing material which ultimately cause arc extinguishment and circuit interruption. All of the above is well known.
Typically, the housing has been made of three types of materials. The housings of some early fuses were made of glass; this type of fuse, the so-called liquid-filled fuse, gave a ready indication of its condition because its interior was clearly visible. For reasons of economy, mechanical strength, and dimensional stability, however, later-developed fuses had housings of fiber-reinforced phenolic materials, of plastics or resins, or of a combination of these. These housings have proved to be more economical, stronger, and as good a dielectric as their glass predecessors. However, since non-glass housings are generally opaque, the condition of a fuse therewithin cannot be readily ascertained. Various schemes were devised, in part to ameliorate this situation.
One such scheme involves a so-called dropout fuse. In this type of fuse, following movement of the arcing rod to extinguish the arc, the arcing rod is driven through and out one end of the fuse to trigger a latching mechanism on an upper fuse mounting. Triggering this latching mechanism permits the fuse to "drop out" and rotate downwardly on a lower hinge mounting, thus giving a visible indication that the fuse has operated.
In another scheme, an end of the arcing rod penetrates and protrudes externally from one end of the fuse following fuse operation to give a visual indication thereof via the protruding arcing rod end. This type of fuse may be termed an indicating fuse.
In a third type of fuse, however--the so-called solid material, non-dropout type--the arcing rod is moved by the pull of a tensioned coil spring. The coil spring coaxially surrounds a flexible cable which electrically connects the arcing rod to an upper fuse ferrule toward which it moves. Thus, the spring and cable are in the path of the movement of the arcing rod and prevent it from performing either a dropout function or an external indicating function. Present-day fuses of the non-dropout variety have opaque housings, as noted earlier. Thus, the condition of such fuses cannot be easily ascertained visually. Conditions of interest include the previous blowing of the fuse or the absence of crucial elements, such as the fusible element or arcing rod. Even with recent developments in the field of plastics and resins, materials electrically and mechanically suitable for use as fuse housings are not sufficiently transparent to permit convenient ascertainment of the condition of the fuse therewithin.